A pressure vessel is made through a meticulous process involving precise cutting, forming, welding, and assembling of materials, typically metals, to create a container capable of safely holding gases or liquids at specified pressures.
Here's a breakdown of the typical manufacturing process:
1. Design and Material Selection
- Design Phase: The design of a pressure vessel must adhere to strict codes and standards (e.g., ASME Boiler and Pressure Vessel Code). This includes calculations for wall thickness, head design, nozzle placement, and safety factors based on operating pressure, temperature, and the contained fluid.
- Material Selection: The selection of the appropriate material is critical. Commonly used materials include carbon steel, stainless steel, and various alloys, each chosen for its strength, corrosion resistance, and weldability.
2. Cutting and Forming
- Plate Cutting: Metal plates are precisely cut to the required dimensions using methods like plasma cutting, laser cutting, or shearing. Accuracy is paramount to ensure proper fit-up during welding.
- Forming: The cut plates are then formed into the desired shape, typically cylindrical or spherical. This is achieved through techniques such as:
- Rolling: For cylindrical shells, plates are passed through rollers to gradually achieve the desired curvature.
- Pressing: For heads (ends of the vessel), hydraulic presses are used to form the plates into hemispherical, ellipsoidal, or other shapes. Heads can also be formed using techniques like cold spinning.
3. Welding
- Welding Procedures: Welding is a critical step, as it joins the formed components to create a leak-tight vessel. Qualified welding procedures must be followed, and welders must be certified.
- Common Welding Methods: Common welding methods include:
- Submerged Arc Welding (SAW): Used for long, straight seams, offering high deposition rates and deep penetration.
- Gas Metal Arc Welding (GMAW/MIG): A versatile process suitable for various materials and thicknesses.
- Gas Tungsten Arc Welding (GTAW/TIG): Provides high-quality, precise welds, often used for critical joints and dissimilar metal welds.
- Weld Inspection: Welds are rigorously inspected using non-destructive testing (NDT) methods such as:
- Radiographic Testing (RT): X-rays or gamma rays are used to detect internal flaws.
- Ultrasonic Testing (UT): High-frequency sound waves are used to detect internal flaws.
- Magnetic Particle Testing (MT): Used to detect surface and near-surface flaws in ferromagnetic materials.
- Liquid Penetrant Testing (PT): Used to detect surface flaws.
4. Nozzle and Fitting Attachment
- Nozzle Installation: Nozzles (inlets and outlets) are attached to the vessel shell using welding. The design and placement of nozzles are critical for proper fluid flow and pressure distribution. Reinforcement pads are often added around nozzle openings to compensate for the reduction in shell strength.
- Fitting Attachment: Various fittings, such as manways, drain connections, and instrument connections, are also welded to the vessel.
5. Post-Weld Heat Treatment (PWHT)
- Stress Relief: PWHT is often required to relieve residual stresses induced by welding. This typically involves heating the entire vessel to a specific temperature and holding it there for a prescribed period, followed by slow cooling. PWHT improves the ductility and toughness of the weldment.
6. Testing and Inspection
- Hydrostatic Testing: The completed pressure vessel undergoes hydrostatic testing, where it's filled with water and pressurized to a value higher than its design pressure. This verifies the vessel's structural integrity and leak-tightness.
- Pneumatic Testing: Pneumatic testing (using air or another gas) can also be performed but is generally avoided due to safety concerns, as a failure can be more energetic than a hydrostatic test. If pneumatic testing is necessary, it requires special precautions and qualified personnel.
- Final Inspection: A final visual inspection is conducted to ensure that all requirements have been met.
7. Painting and Finishing
- Surface Preparation: The vessel's exterior surface is prepared by sandblasting or other methods to remove mill scale, rust, and other contaminants.
- Painting/Coating: A protective coating system is applied to prevent corrosion and enhance the vessel's appearance. The specific coating is selected based on the operating environment and the material of the vessel.
8. Documentation
- Manufacturing Data Report: A comprehensive Manufacturing Data Report (MDR) is compiled, documenting all aspects of the vessel's fabrication, testing, and inspection. This MDR serves as a permanent record of the vessel's compliance with applicable codes and standards.
